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Surveys of Potential Yellow Fever Vectors in Gabon And

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Surveys of Potential Yellow Fever Vectors in Gabon And WORLD HEALTH ORGANIZATION'I WHO/VBC/71.279 I I ORGANISATION MONDIALE DE LA SANT6 ORIGINAL: FRENCH i .SURVEYSOF POTE~IALYELLOW FEVER VECTORS IN GABON AND CHAD (21 October - 7 November 1970) by 1 J. MOUCHET " 4 ' i. AIMS Little is known regarding the distribution and ecology of the oulicinae especially of potential yellow fever vectors in Gabon and Chad, whereas information is relatively more plentiful concerning the other States belonging to the OCEAC, namely Cameroon, the C.A.R. and Congo. Consequently the author took advantage of a stay of two weeks in Gabon to study the ecological conditions and quantitative distribution of A. aegypti in the north-west of the country. During three days spent in Chad, similar studies were restricted to a few villages in the neighbourhood of Fort Lamy. At the same time, certain data were collected on culicid fauna in Gabon. In addition, seven A. aegypti strains were collected 5n Gabon for laboratory breeding and subsequent investigation of their susceptibility to insecticides, an essential guide in any control campaign. This research was motivated by the concern of OCEAC, which has been aroused by the recrudescence of yellow fever in Africa and was therefore welcomed by the authorities of that Organization, which is endeavouring to promote a rational prophylaxis of the endemic in the Member States. 2. SURVEYS IN GABON 2.1 Epidemiological and entomological background Memories of the devastating yellow fever epidemics are still vivid in the coastal region of Gabon, but no case of the disease has been reported since 1949. An OCEAC information document (June 1970) mentions the presence of antibody in a few unvaccinated subjects and does exclude the possibility of jungle transmission in the east of the country. The list of mosquitos in Gabon (Annex 1) has been drawn up on the basis of the work of Edwards, Galliard, Grjebine, Lacan and De Meillon. Although it is fairly extensive as regards the vector mosquitos of malaria, it is still very brief as regards the culicinae and especially Aedes, the potential yellow fever vectors. In view of the richness in species of the neighbouring forest areas, it is clear that there are enormous gaps in our knowledge of the entomological fauna of Gabon. Moreover, the fact that certain species which are plentiful everywhere in the forest, such as Erematpodites chrysogaster, have not been previously reported is rather significant of this state of affairs. Medical Entomologist, Central Scientific Services of ORSTOM, 70-74 route d'Aulnay, 93 Bondy, France. The issue of this document does not constitute Ce document ne constitue pas une publication. formal publication. It should not be reviewed, II ne doit faire l'objet d'aucun compte rendu ou abstracted or quoted without the agreement of resume ni d'aucune citation sans l'autorisation de the World Health Organization. Authors alone l'Organisation Mondiale de la. a te.Q% les op¡Q¡$@@, are responsible for views expressed in signed exprimees dans les artidksw &.a sighds deGagent6;- WHO/VBC/71.279 Page 2 2.2 Organization of the surveys An attempt was made to visit as many localities as possible in the time available, i.e. 12 days. For this purpose it was necessary to give preference to towns and villages situated on main roads accessible at that period of the year (12 localities in west Gabon). As a consequence, villages isolated in the bush away from roads passable by vehicles could not be reached. The author had at his disposal a team limited to a sanitation technician and two operators, personnel kindly made available by the Libreville Urban Hygiene Service. Activities were centred on the search for larval breeding places, omitting captures of adult mosquitos, despite their value, because of lack of time and especially of trained personnel. .In each locality a certain number of houses and their surroundings were visited; they were chosen in different neighbourhoods so as to obtain a representative sample. In very small hamlets all dwellings were examined. In addition, samples were taken in garages (tyres), stores of equipment and materials (old iron), and natural breeding places (tree- holes, the axils of plants with ensheathing leaves). The specimens of larvae collected were bred in an improvised laboratory for rapid classification of the adults or in order to obtain egg batches. Surplus specimens preserved in alcohol were classified later on, especially Culex. 2.3 Situation as regards- Aedes aegypti L. West central Gabon is under the influence of an equatorial climate with four seasons. The rainfall, which is greater than 2000 mm, is spread over two rainy seasons while there are also two shorter dry seasons. The vegetation is of the equatorial forest type, and thins out to some extent around villages and forestry work-sites. This survey took place at the beginning of the rainy season (late that year). It was raining hard everywhere except at Port Gentil, wherb the dry season stil.1 continued, Table I summarizes the captures made and shows the presence of A. aegypti in all the villages visited except one which was recently built (Oyan IV). ‘However it cannot be concluded that this mosquito is present in all the villages of west Gabon since villages in the bush away from tracks passable by motor vehicles were not reached. But, as will be .I shown later on, it is precisely in such villages that the species might be absent. In the towns and villages of the Gabonese forest, A. aegypti is a peridomestic insect but not a domestic one. Hardly any larvae are found in stores of water inside dwellings since they are small and frequently renewed. On the other hand, larvae are not uncommon in drums placed outside dwellings to collect rain water from the roof and store it for domestic use. Such containers even constituted the few rare breeding places at Port Gentil. In general, discarded containers outdoors (cans and various receptacles, old tyres, scrap iron, etc.) comprise the main A. aegypti breeding places in Gabon. This waste of industrial civilization accumulates behind the houses; its volume depends on the purchasing power of the population but is inversely porportional to the activities of the garbage clearance services. Larvae were found only once in a natural biotope (tree-holes). Quantitative evaluation of A. aegypti populations using the house index is difficult . ,4 since the larvae are practically always found outdoors. The container index (percentage of containers holding larvae) is more revealing, but it is the Breteau index (number of breeding places per 100 houses) in particular which seems most representative. The Breteau index which is 43 for the Libreville built-up area was generally more than 20 in the localities visited, except at Port Gentil where the climatic conditions were exceptional for the season. WHO/VBC/71.279 Page 3 The situation as regards A. aegypti in the forest area of Gabon is comparable to that observed in the Ivory Coast, where that mosquito is plentiful in the towns, forest camps and areas with an industrial income, Nevertheless, a true picture of the situation would necessitate the survey of villages further away from the towns and industrial areas, a programme which could not be carried out because of lack of time. The’ A. aegypti strains collected all bite man very readily but no information exists as concerns their aggressiveness towards that host under natural conditions. The author was able to observe only that the females entered houses and bit severely at dusk in Libreville. The observation Qf the presence of A. aegypti larvae in drums holding water at the end of the dry season at Port Gentil would seem to show that the species exists throughout the year in hoth the larval and adult states, since this type of breeding place is permanently present in all the localities surveyed. 2.4 Other potential vectors in Gabon Among the other .potential yellow fever vectors only Aedes simpsoni was known in Gabon; its larvae were found at Oyan in one of their preferred biotopes, namely the axils of banana tree leaves. In addition Aedes africanus was found in cut bamboo at Cape Esterias. These are common species which must certainly exist all over the country. Another experimental yellow fever vector is very plentiful in Gabon, namely Eretmapodites chrysogaster. Its larvae, or”those of other species of this group, are constantly found in containers thrown away in the undergrowth around villages. However, there is no information concerning the density, aggressiveness and anthropophily of these mosquitos in Gabon. This is a task calling for a trained team, which was not at the author’s disposal. 2.5 Epidemiological considerations Although there have for long been no epidemics, the area of west cen.tra1 Gabon surveyed should be regarded as epidemiologically dangerous because of the high A. aegypti density, for it is felt.that yellow fever can take on epidemic proportions when the Breteau index is greater than 10 which is the case here. It follows that vaccination is necessary all over this area. The possibility of forest transmission of the virus in the bush has been considered following the discovery of antibody in unvaccinated children (Lourie). It is certain that more extensive and more thorough surveys are called for among the vectors, the potential reservoirs and man to detect possible jungle foci. The discovery of a case in the forest area‘of Cameroon (Ayos) can but add to the interest of such investigations in Gabon. 2.6 Control methods A. aegypti control in Gabon can only be conceived of as a sanitary operation, because of the temporary and peridomestic nature of the breeding places. The elimination of tin cans, other containers and old tyres would certainly reduce the rates to below the danger threshold.
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